1. Field of the Invention
The present invention relates to a method and apparatus for electrophotographic image forming. In particular, the present invention relates to a method and apparatus for electrophotographic image forming capable of effectively performing an image transfer operation.
2. Discussion of the Background
Recent market trends indicate a strong demand for electrophotographic image forming apparatuses having a function of printing color images. In response to the market demands, the availability of color electrophotographic image forming apparatuses, such as color copying machines and color printers, has remarkably increased.
The color electrophotographic image forming apparatuses can generally be classified into two types, that is, a one drum image forming apparatus and a tandem image forming apparatus.
The one drum image forming apparatus includes a photoconductive element having a plurality of image developing units around the photoconductive element. These image developing units electrically hold respective toners of different colors to sequentially form each of respective toner images on a surface of the photoconductive element. These respective toner images are then overlaid onto a recording sheet so that a full-color image is formed.
The tandem image forming apparatus includes a plurality of photoconductive elements and a plurality of developing units corresponding to the plurality of respective photoconductive elements. The plurality of developing units develop respective color toner images of different colors on the plurality of respective photoconductive elements. These color toner images are sequentially transferred onto a recording sheet to form a full-color image.
When comparing the one drum image forming apparatus and the tandem image forming apparatus, the following characteristics may be discerned.
The one drum image forming apparatus has an advantage that using only one photoconductive element makes a device relatively compact and inexpensive. However, the one drum image forming apparatus, which has one photoconductive element, needs to repeat its image forming operation several times (generally four times) to develop a full-color image. This process consumes a considerable amount of time.
The tandem image forming apparatus has an advantage that a plurality of photoconductive elements can reduce a time period of the image forming operation. However, the plurality of photoconductive elements make an image forming apparatus larger and more expensive.
Since the market requires a full-color image forming apparatus that performs its image forming operations at a speed equivalent to a monochrome image forming apparatus, the tandem image forming apparatus is attracting attention.
A tandem image forming apparatus includes a direct transfer system or an indirect transfer system.
In the direct transfer system, a plurality of photoconductive elements are arranged in parallel with a surface of a sheet transfer belt that forms an endless belt, and a plurality of transfer units having respective colors of yellow (y), magenta (m), cyan (c), and black (bk) are disposed in a vicinity of the plurality of respective photoconductive elements. Respective color toner images formed on surfaces of the plurality of photoconductive elements are sequentially transferred by the plurality of transfer units onto a recording sheet that is conveyed by the sheet transfer belt.
In the indirect transfer system, a plurality of photoconductive elements are arranged in parallel with a surface of an intermediate transfer member forming an endless belt. Respective color toner images formed on surfaces of the plurality of photoconductive elements are sequentially transferred and overlaid by a plurality of respective primary transfer units onto a surface of the intermediate transfer member so that an overlaid color toner image is formed. Subsequently, a secondary transfer unit transfers the overlaid color toner image onto a recording sheet. The secondary transfer unit may employ a transfer belt system or a roller system.
In the transfer belt system and the roller system, it has been a significant challenge to overlay a plurality of color toner images having different colors onto a transfer member without color shift. To achieve the above-described purpose, attempts have been made to rotate the sheet transfer belt and the intermediate transfer member at a constant rate.
One solution is to measure a surface speed with a linear encoder. The linear encoder performs a feedback control based on its output and uses the output to adjust the rate for writing. This system may efficiently be used to achieve accurate alignment.
It is, however, difficult to effectively form the linear encoder on an endless belt. In a case in which the endless belt has a surface that is clear and transparent, encoder marks may be printed on the clear surface of the endless belt so that the linear encoder can read the encoder marks to measure a surface speed of the endless belt. However, since transferring images needs a predetermined amount of conductivity, the endless belt should include conductive materials such as carbon material, which prevents the surface of the endless belt from being clear and transparent. As an alternative to the clear surface, a reflective linear encoder may be provided on the endless belt. The reflective linear encoder generally includes a metal etching or printing layer to obtain high reflectance.
The reflective linear encoder should be carefully positioned when it is disposed in a vicinity of a transfer unit that includes components having electrically high voltage such as a bias roller. When a creepage distance between the reflective linear encoder and the transfer unit is not sufficiently maintained, high voltage may leak from the transfer unit to the metal layer of the linear encoder, causing electromagnetic noises and deterioration in image quality. The above-described problems may occur in a tandem image forming apparatus and a one drum image forming apparatus, and should be solved to obtain images having higher quality.